In the interest of improving cancer treatment, considerable attention has been placed on the modification of radiation damage. The major goal of this project continues to define and understand those aspects of unique tumor physiology, anatomy, cellular, and molecular processes that ultimately define the very nature of a tumor such that a particular dose of ionizing radiation, when used will be more effective. One means to that end is to investigate the interaction of ionizing radiation with a variety of chemotherapy agents to assess if tumors can be made more sensitive. We previously demonstrated that treatment of diverse human tumor cell lines with paclitaxel induced a block in G2/M of the cell cycle (radiosensitive phases of the cell cycle) which resulted in significant radiosensitization. As a consequence, translation of this pre-clinical information into a clinical trail combining radiation and paclitaxel in the treatment of head/neck cancer is currently underway in the Radiation Oncology Branch. We have tested other chemotherapeutic agents including MGI-114, a new alkylating sesquiterpenoid which has shown cytotoxicity in both in vitro and in vivo models. We have shown that MGI-114 selectively radiosensitizes human breast and colon cell lines to radiation with an indication that the sensitization is related to cell cycle blocks in S phase. Likewise, another novel agent is being evaluated for radiosensitization properties. Ecteinascidin 743, an agent with anti-tumor activity that selectively alkylates guanine N2 is currently being evaluated in cell culture. Another major thrust of this project is to develop functional imaging techniques to better characterize factors important in the tumor microenvironment that may prevent or diminish agents from impacting radiation response. It is well established that hypoxia is a major determinant of radiation sensitivity. Therefore, we are using several murine tumor models to study tumor hypoxia. Our approach is to use current invasive techniques and extend that information to non-invasive methods that are under development, such that patient tumor treatment profiles may optimized on an individual basis. To that end we are using in vitro and in vivo survival techniques, oxygen probes, nitroimidazole fixation followed by immunohistochemistry, electron paramagnetic resonance (EPR), and Overhauser MRI (OMRI). Presently we are comparing/contrasting these various techniques with the goal of ultimately bringing non-invasive EPR or OMRI functional imaging to clinical trial. The information we acquire will teach us how to assess oxygen status in a tumor in a non-invasive manner to effect optimum treatment. We have recently compared oxygen profiles in mouse tumors collected by OMRI and compared the results with conventional oxygen electrode measurements. OMRI oxygen profiles correlate quite well with oxygen electrode measurements, thus validating this non-invasive technique for oxygen concentration assessment in tissues. Additionally, we are developing a technique of non-invasive redox mapping of tissues using EPR and a stable paramagnetic probe (nitroxide). Collectively, these non-invasive functional imaging approaches should enhance our ability to better understand the tumor microenvironment and develop strategies to effectively attack potential barriers that currently limit the effectiveness of cancer treatment modalities.